Rapier looms are known and are distinguished from other types of loom by the nature in which the yarn is moved through the warp threads. On a traditional shuttle loom a quantity of yarn is carried on the shuttle and the assembly is projected through the warp threads to the other side of the loom.
The primary benefit of a rapier loom is that the yarn is pulled from the main spool, reducing waste and increasing speed. In order to maintain stiffness of the insertion part and the speed of weaving, there are two rapiers on the loom, and the yarn is handed over at the centre of the shed. The yarn is fed by a part commonly termed the giver and is collected by a part termed the taker.
In order to effect a consistent weave it is essential that the yarn is held reliably and can be freely released by the giver at the centre. Some rapiers, termed negative rapiers, use vee slots to capture the yarn and rely upon friction to hold it during the insertion process. Other devices, most specifically those manufactured by Dornier, are termed positive rapiers and use a spring clamp that is opened at the centre of the loom by external fingers pressing upon suitable release faces.
Negative rapiers have two key disadvantages. Firstly, not all yarns can be woven, due to the relationship of friction and fibre stiffness. Secondly, the yarn must travel through the vee during the capture process so high wear can occur, resulting in material being wasted.
Positive rapiers have the disadvantage that the insertion parts are of high mass causing the maximum speed of weaving to be much lower, resulting in productivity losses. The external fingers can also cause damage to the warp threads, creating an imperfection in the yarn.
It is impractical to mount a stored energy system such as a capacitor and solenoid upon the rapier arms, because the energy density to mass ratio is very poor. In any high speed reciprocating system, mass is a serious impediment, because acceleration will decrease for a given force as the mass increases, from F=ma. Any energy store will greatly increase the inertia in the system.
In a negative rapier loom the heads can accelerate at up to 5000 ms−2, equivalent to 500 gravities. Any weight therefore acts as if it is 500 times heavier. To keep the clamps of a positive rapier closed under such forces stiff springs are used. The springs act like masses and so slow down the opening at the centre of the loom, limiting the operating speed further.
It will be beneficial to eliminate the need to overcome the closure springs at the time the system condition needs to be changed. It is further beneficial to reduce the force needed to close upon and retain the yarn.
It is further a feature of high speed devices that the forces within the system cause elongation and associated shape changes in the working parts. Some rapier systems use rods to insert the heads whereas others use stiff belts, preferably reinforced with carbon fibre. These belts elongate under the acceleration, changing the physical relationship of the heads, and so making the handover process speed dependent. Setting up the loom at low speeds requires consideration of the behaviour of the belts and anticipation of the weave speed. It will be beneficial to be able to dynamically alter the loom timing by some electronic means.